Add org.wildfly.security:wildfly-elytron-http-external dependency

The org.wildfly.security:wildfly-elytron-http-external dependency is required for the Elytron component upgrade containing the HTTP EXTERNAL mechanism. Without this added, the following error will occur when building Core:

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[WARNING] Rule 0: org.apache.maven.plugins.enforcer.BanTransitiveDependencies failed with message:
org.wildfly.core:wildfly-core-feature-pack-common:pom:13.0.0.Beta6-SNAPSHOT
org.wildfly.security:wildfly-elytron-http-deprecated:jar:1.13.0.Final:compile has transitive dependencies:
org.wildfly.security:wildfly-elytron-http-external:jar:1.13.0.Final:compile
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